Dietary fiber is defiined as the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine, with complete or partial fermentation in the large intestine. (Dietary Fiber Definition Committee).

Fiber consists of undigested plant carbohydrates in both soluble and insoluble forms.

Soluble fiber dissolves in water to form a gel, while insoluble fiber does not.

While dietary fiber is not digested by human enzymes, some fibers may be metabolized by colonic bacteria producing compounds that can affect the colonic mucosa and the diversity of the bacterial population in the microbiome.

Both types of fiber increase stomach distension, slows nutritional absorption and increases satiety (Howarth NC et al).

Fiber is thought to enhance satiety by adding bulk and viscosity to gut contents and altering gut hormones. 

Fiber plays a significant role in blood sugar management by slowing down the rate that carbohydrates break down, and the rate that the body absorbs the resulting sugars.

Potential fiber protective mechanisms include dilution of fecal carcinogens, reduced transit time of feces, binding of carcinogenic bile acids, and the production of short chain fatty acids.

Short chain fatty acids are the main energy compounds in colonic epithelial cells.

Dietary Fiber Intake Shows Inverse, Dose-Dependent Risk Ratio with CHD, CVD

Dietary fiber intake has been linked to a lower incidence of coronary heart disease.

Postulated dietary fiber decreases absorption of low-density lipoprotein (LDL) from the intestines, increases satiety resulting in less weight gain.

Dietary fiber in food promotes satiety and reduced food intake and may explain the association between fiber intake and lower body weight. 

Antioxidants in high fiber foods that may have a beneficial effect.

There is a lower risk of CVD and CHD associated with higher intake of insoluble, cereal, fruit, and vegetable fiber.

The most common natural sources of fiber include fruits and vegetables as well as wheat or oat bran, and are the most likely to cause flatulence.

Fiber is made by plants and is not easily digested by the human gastrointestinal tract.

Soluble fiber is prebiotic and readily fermented in the colon into gases, while insoluble fiber is metabolically inert and absorbs water as it moves through the digestive system, aiding in defecation.

Most types of insoluble fiber attach to body water in the intestine and increase the volume of stools.

Increasing dietary fiber can be accomplished by eating whole grains, beans, lentils or by consuming fruits and vegetables daily.

Soluble, and to a lesser extent insoluble, fiber are fermented by intestinal bacteria to produce short chain fatty acids which affect hepatic insulin sensitivity and lipid synthesis (Howarth NC et al).

Insoluble fiber’s main function is to increase fecal bulk.

Increases bulk and reduces transit time of stool through the bowel, increases the excretion of bile acids, estrogen, and fecal procarcinogens and carcinogens by binding to them, lowers serum cholesterol, slows glucose absorption, improves insulin sensitivity, decreases blood pressure, promotes weight loss, inhibits lipid peroxidation, and has anti-inflammatory properties.

Studies suggest fiber intake associated with a 16% reduction in colorectal cancer risk in the highest quintile group compared to the lowest, but when aadjusting for age and nutritional and lifestyle factors, only a weak inverse association remained.

Recommended that men age 50 and younger consume at least 39 grams of fiber per day, and women of the same age consume at least 25 grams per day.

The Dietary Guidelines for Americans recommends at least 25-30 grams of fiber.

The best sources of fiber include fresh fruits, vegetables, beans, nuts, corn, and foods containing wheat bran.

Dietary fiber lowers blood lipid levels and glucose levels, in part by restricting their absorption from the gastrointestinal tract.

Dietary soluble fiber increases the production of cholesterol binding bile acids which promotes excretion of lipids.

Fiber may lower blood pressure by lowering insulin levels.

Diets high in total fiber lowers systolic and diastolic pressures in hypertensive patients compared with low fiber diets (Whelton SP et al).

Meta-analyses show diets high in soluble fiber such as oats and psyllium, reduced total and low-density lipoprotein cholesterol levels compared with diets low in soluble fiber or high in soluble fiber ( Brown L et al).

Fiber inversely associated with type II diabetes in meta-analyses of prospective cohort studies, although the effect was confined to cereal fiber rather than fruit or vegetable fiber ( Schultze MB et al).

Cereal fiber is an important source of magnesium, a cofactor for tyrosine kinase and is active in insulin signaling.

Navy beans-half cup has 9.5 grams, kidney beans half a cup 8.2 grams, pinto beans half a cup 7.7 grams, black beans half a cup 7.5 grams, lima beans half a cup 6.6 grams, white beans half cup 6.2 grams, artichoke 1 6.5 grams, sweet potatoes 1 medium size 4.8 grams, pears, 1 small pear 4.4 grams, green peas half cup 4.4 grams, raspb2242ies half a cup 4.0 grams, blackb2242ies a half cup 3.8 grams, prunes half a cup 3.8 grams, figs and dates quarter cup 3.6 grams, spinach a half cup 3.5 grams, apples 1 medium 3.3 grams and oranges 1 medium 3.1 grams.

Soluble fiber is in the following foods:

vegetables legumes whole grains fruit

Scottish Heart Health study found dietary fiber intake is inversely related to total mortality in men but not women (Todd S et al)

Zutphen Study found a 9% lowered risk of total death per 10 gm/d of dietary fiber intake.

In an Israeli study a 43% lowered risk of total death in persons consuming 25 g per day or more of dietary fiber compared with those with less than 25 g per day of diet fiber intake (Lubin F et al).

In the National Health and Nutrition Examination Survey I Epidemiologic Follow-up Study found no association between dietary fiber intake and total mortality, but utilized a dietary fiber assessment intake with a single 24 hour dietary recall process(Bazzano LA et al).

The NIH-AARP studied examining dietary fiber intake in relation to total mortality and death from specific causes, a prospective cohort study of nine years average duration of follow-up identifying 20,126 deaths in men and 11,330 deaths in women: dietary fiber significantly lowered the risk of total death in both men and women, and also lowered the risk of death from cardiovascular, infectious, and respiratory diseases by 24%-56% in men and 34%-39% in women (Park Y et al).

Dietary fiber especially from grains is inversely associated with the risk of death from infectious and respiratory diseases.

Dietary fiber has anti-inflammatory properties with lower levels of inflammatory markers such as C. reactive protein, IL-6, tumor necrosis factor alpha receptor 2.

In the above study there was an inverse relationship between dietary fiber intake and cancer death in men but not in women and dietary fiber from grains but not from other sources, was significantly inversely related to total and cause specific death in both men and women.

Pooled analysis of 10 perspective studies estimated the risk of cardiovascular disease death decreased by 19% per 10 g per day increment of dietary fiber intake, And that fiber from both grains and fruits was associated with a lowered risk (Periera MA et al).

Dietary fiber intake probably lowers the risk of colorectal and esophageal cancers.

The Iowa Women’s Health Study found that women who consume large amounts of whole grains had a 34% lower risk of death from non-cardiovascular, noncancer inflammatory diseases and a 40% lower risk of death from respiratory diseases (Jacobs DR et al).

Whole grains have high antioxidant potential and contained numerous minerals including selenium and zinc which are antioxidant enzyme system cofactors, which may protect tissues from oxidative damage that characterizes chronic inflammation.

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